1. Field of the Invention
The present invention relates to a multi-piece solid golf ball comprising a multi-layered core and a polyurethane cover covering the core.
2. Description of the Related Art
A conventional solid golf ball has a solid core made from vulcanized rubber and an ionomer resin cover because an ionomer resin cover exhibits superior durability. Golf balls with ionomer resin covers, however, give golfers a larger impact upon shot than golf balls with Balata rubber covers and hence are likely to impart the golfers with inferior shot feeling.
In attempt to improve the shot feeling imparted by golf balls with the ionomer covers, Japanese Patent No. 2709950, for example, has proposed a cover made from a mixture of a hard ionomer such as a sodium salt or zinc salt of an olefin-unsaturated carboxylic acid copolymer and a soft ionomer such as a sodium salt or zinc salt of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester terpolymer. The cover makes it possible to render the shot feeling soft due to the soft ionomer in the cover material, but on the other hand, impairs the merits inherent to ionomer covers; for example, the repulsion property as well as the scuff resistance of the golf ball lowers.
In recent years, polyurethane is receiving attention as an inexpensive cover material that imparts to golfers a shot feeling analogous to a shot feeling imparted by the Balata cover and has higher durability than the Balata cover. U.S. Pat. No. 5,334,673, for example, has disclosed a polyurethane cover formed by curing a urethane prepolymer with a slow-reactive polyamine curing agent. Also, Japanese Patent Laid-Open Gazette No. HEI 9-215778 has proposed a cover made from thermoplastic polyurethane.
Polyurethane used as a cover material involves a problem that it makes the molding of a cover difficult because the curing reaction between a urethane prepolymer and a polyamine proceeds rapidly, which causes a steep increase in viscosity. The U.S. Pat. No. 5,334,673 has solved the problem by the use of a slow-reactive polyamine and/or a glycol. In the art of the publication, however, there are some cases depending on kinds of urethane prepolymer, curing agent, and/or combinations thereof in which an increase in viscosity is still rapid and makes the molding of a cover difficult. Even though a polyurethane cover is obtained by molding, a golf ball with the resulting cover is not sufficient in terms of repulsion property, spin performance and scuff resistance. Thus, further improvements with respect to a thermosetting type polyurethane cover are desired.
Thermoplastic polyurethane elastomers used as cover materials are superior in moldability to thermosetting polyurethane covers, but are inferior in wear resistance, tear strength and scuff resistance to thermosetting polyurethane covers or hard ionomer covers because of the lack of three-dimensionally crosslinking points.
An object of the present invention is to provide a solid golf ball with a cover made from polyurethane which is satisfactory in moldability and ball characteristics.
A solid golf ball of the present invention comprises a solid core comprising a solid center made from vulcanized rubber and an intermediate layer covering the solid center, and an cover covering the solid core. The cover has an outermost layer made from polyurethane obtained by curing a composition comprising an isocyanate group-terminated urethane prepolymer and an aromatic polyamine curing agent. The isocyanate group-terminated urethane prepolymer has a residual polyisocyanate monomer content of not more than 0.1% by mass.
The foregoing and other objects, features and attendant advantages of the present invention will become apparent from the reading of the following detailed description.